Electrochemical cell monitoring and balancing circuit with self-diagnostic feature

ABSTRACT

A system and method for measuring a voltage of electrochemical cells of a pack. The system includes circuit elements individually associated with respective electrochemical cells of the pack and having electrical characteristics that are different such that individual electrochemical cells can be distinguished from one another. The system also includes a measurement circuit configured to measure the voltage of the electrochemical cells and to identify an electrochemical cell being measured based on an electrical characteristic of a circuit element associated with the electrochemical cell. Various self-diagnostic techniques are described, as well as techniques for measuring sense resistance, reducing sense resistance, and measuring changes in voltage of a cell over time.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/312,025, filed Dec. 6, 2011, titled “ELECTROCHEMICAL CELL MONITORINGAND BALANCING CIRCUIT WITH SELF-DIAGNOSTIC FEATURE,” which claims thebenefit under 35 U.S.C. 119(e) of U.S. Provisional Application No.61/420,259, filed Dec. 6, 2010, titled “ELECTROCHEMICAL CELL MONITORINGAND BALANCING CIRCUIT WITH SELF-DIAGNOSTIC FEATURE,” U.S. ProvisionalApplication No. 61/420,261, filed Dec. 6, 2010, titled “ELECTROCHEMICALCELL BALANCING CIRCUITS AND METHODS, and U.S. Provisional ApplicationNo. 61/420,264, filed Dec. 6, 2010, titled “SYSTEM AND METHOD FORMEASURING ISOLATED HIGH VOLTAGE AND DETECTING ISOLATION BREAKDOWN WITHMEASURES FOR SELF-DETECTION OF CIRCUIT FAULTS, each of which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

Described herein are methods and systems for charging and discharging ofelectrochemical cells and, more specifically, to methods and systems formonitoring and managing the charging and discharging of electrochemicalcells.

BACKGROUND

The need for monitoring and managing large arrangements ofelectrochemical energy storage cells for various applications is known.Systems for performing such monitoring and management typically includefeatures such as voltage measurement, temperature measurement andbattery cell balancing (i.e. equalization) either through selective celldissipative discharging or charge redistribution. Of particular interestare large-scale systems used for storing electrical energy for thepropulsion of vehicles as well as energy storage systems for electricalgrid support and supplying power to remote locations.

Various methods exist for attempting to verify the measurement data byexamining its content to determine the plausibility of the informationgathered. One method is to compare the sum of the cell voltages to thepack voltage. However, this method needs extremely accuratemeasurements. A 200 mV error might seem very small for a pack voltage,while it can be very large for a cell voltage. Other methods includechecking the ranges of the measurements of the cells to ensure that theyseem reasonable. This technique could catch issues where a reading islow enough or high enough to be unreasonable, but would not catch anyerrors in which a cell reading is replaced by a different cell'sreading. Moreover, modern batteries have an increasingly minimal changein cell voltage as a function of depth of discharge, meaning that it isquite normal that a number of cells will appear to be (within theprecision of the measurement system) at the same voltage over a largerange of a system's lifetime.

A second method is known in the art, which suggests using two redundantmeasurement systems. However, this method may add significant cost tothe monitoring solution.

Due to the architecture of a number of measuring circuits, there isoften a great deal of shared componentry used to measure a number ofcell voltages or temperatures. This is done to reduce the costassociated with large numbers of highly accurate components that a lackof sharing would impose, especially when a system contains a high numberof cells.

Most large-format battery systems operated over a long period of timerequire cell balancing. This can be accomplished by selective cellcharging, selective cell discharging, charge shuttling, or combinationsof the above. A number of methods exist for these strategies.

SUMMARY

Using the systems or methods described herein, cells of a battery packmay be identified and faults within measurement, selection and/orbalancing circuitry for a battery pack may be detected.

Some embodiments relate to a system for measuring a voltage ofelectrochemical cells of a pack. The system includes circuit elementsindividually associated with respective electrochemical cells of thepack and having electrical characteristics that are different such thatindividual electrochemical cells can be distinguished from one another.The system also includes a measurement circuit configured to measure thevoltage of the electrochemical cells and to identify an electrochemicalcell being measured based on an electrical characteristic of a circuitelement associated with the electrochemical cell.

Some embodiments relate to a method of monitoring an electrochemicalcell of a pack comprising a plurality of electrochemical cells. Themethod includes measuring a voltage of the electrochemical cell andidentifying the electrochemical cell from among the plurality ofelectrochemical cells based on an electrical characteristic of a circuitelement connected to the electrochemical cell.

Some embodiments relate to a system for monitoring and balancingelectrochemical cells of a pack. The system includes a balancing circuitconfigured to charge or discharge a first electrochemical cell of thepack. The balancing circuit is configured to generate a current throughthe first electrochemical cell. The system also includes a measurementcircuit connected to the first electrochemical cell, the measurementcircuit being configured to measure, based on the current, a senseresistance established by coupling the first measurement circuit to thefirst electrochemical cell.

Some embodiments relate to a system for monitoring electrochemical cellsof a pack. The system includes a measurement circuit connected to afirst electrochemical cell of the pack and a current generating circuitconfigured to generate a wetting current through the firstelectrochemical cell of sufficient magnitude to wet a contact betweenconductors coupling the measurement circuit to the first electrochemicalcell.

Some embodiments relate to a method of monitoring an electrochemicalcell. The method includes measuring a change in voltage over a timeperiod as the electrochemical cell charges or discharges anddetermining, based at least partially on the change in the voltage overthe time period, a range of the state of charge of the electrochemicalcell.

Some embodiments relate to a system for monitoring an electrochemicalcell. The system includes a measurement circuit configured to measure achange in voltage over a time period as the electrochemical cell chargesor discharges. The system also includes a controller configured todetermine, based at least partially on the change in the voltage overthe time period, a range of the state of charge of the electrochemicalcell.

The foregoing summary is provided by way of illustration and is notintended to be limiting.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary battery monitoring and balancing system.

FIG. 2 shows a schematic of an embodiment of a battery monitoring andbalancing system including resistors in series with each cell which mayhave different values to differentiate cells from one another.

FIG. 3 shows a schematic of a circuit in which a current source chargesor discharges a cell group through the series combination of a cell anda resistor.

FIG. 4 shows a schematic of a circuit in which a voltage source isconnected to charge or discharge a cell group.

FIG. 5 shows a schematic of an embodiment in which bipolar transistorsdraw current from the cells.

FIG. 6 shows an embodiment in which seven modules are connected togetherto form a device capable of measuring twenty-eight cells.

FIG. 7 shows an exemplary plot of cell voltage vs. state of charge(SOC).

DETAILED DESCRIPTION

Described herein are techniques and circuits for monitoring batterycells than can include the capability of diagnosing malfunctions withthe battery measurement system or battery balancing system. Suchcircuits and techniques can verify that measurements are indeed from theintended cells, and can verify that the balancing circuit is operatingas intended.

An exemplary battery monitoring and balancing system will be describedwith reference to FIG. 1. FIG. 1 shows an exemplary battery monitoringand balancing system connected to a battery pack 2. As shown in FIG. 1,the battery pack 2 may include a plurality of electrochemical cells,CELL1, CELL2, CELL3, and CELL4. Although only four cells are shown inFIG. 1, it should be appreciated that a battery pack such as batterypack 2 may have any suitable number of cells. The cells of battery pack2 may be connected in any suitable manner, such as in series, inparallel, or any suitable combination thereof. The number and/topologyof cells in battery pack 2 may be selected based on the voltage, currentand/or battery capacity desired for a particular application. Exemplaryapplications for the battery pack 2 include electric vehicles, hybridvehicles, or grid-tied energy storage systems. However, the techniquesand circuits described herein are not limited to use in vehicles orgrid-tied energy storage systems, as they may be applied to any suitableapplication.

The battery monitoring and balancing system shown in FIG. 1 includes amonitoring system 4 connected to the plurality of electrochemical cellsin the battery pack 2. The monitoring system 4 may be any suitablemonitoring system of having any of a variety of topologies, including,but not limited to switched capacitor, multiplexer, switches connectedto an analog-to-digital converter (ADC), or combined multi-leveltopologies of multiplexers in front of switches. The monitoring system 4may have a first terminal connected to the positive terminal of eachcell and a second terminal connected to the negative terminal of eachcell. For example, as shown in FIG. 1, terminal SP0 of the monitoringsystem 4 is connected to the negative terminal of CELL1 and terminal SP1of the monitoring system 4 is connected to the positive terminal ofCELL1. Terminal SP1 of the monitoring system 4 is connected to thenegative terminal of CELL2 and terminal SP2 of the monitoring system 4is connected to the positive terminal of CELL2. Terminal SP2 isconnected to the negative terminal of CELL3 and terminal SP3 isconnected to the positive terminal of CELL3. Terminal SP3 is connectedto the negative terminal of CELL4 and terminal SP4 is connected to thepositive terminal of CELL4. A selected cell or group of cells can bemonitored by selecting the appropriate terminals of monitoring system 4at which to receive input. For example, terminals SP0 and SP1 can beused to monitor CELL1. Such monitoring may include measuring the voltageof CELL1 and/or the current through CELL1. As another example, thevoltage and/or current of the entire battery pack 2 can be measuredusing terminals SP0 and SP4. The monitoring system 4 may select anysuitable cell or combination of cells in battery pack 2 for monitoring.Monitoring system 4 may perform a sequence of measurements of differentcells and/or combinations of cells in accordance with a monitoringprogram executed by a controller.

The battery monitoring and balancing system shown in FIG. 1 alsoincludes a balancing system 6 for charging and/or discharging the cellsof battery pack 2. The balancing system 6 may be configured to maintainthe cells of the battery pack 2 at a suitable voltage and/or chargelevel to prolong their usable life (e.g., by avoiding over-charging orover-discharging cells). Balancing system 6 may be configured to performsuch balancing in response to a balancing program executed by acontroller. The balancing system 6 may be any suitable balancing systemof any of a variety of topologies, including, but not limited toswitches to a shared discharge or charge circuit, a switched-in bleedingresistor, a switched charge/discharge current source, a charge/dischargecurrent source that can be set to zero current, or switches to a voltagesource. The balancing system 6 includes a plurality of balancingcircuits 6 a, 6 b, 6 d and 6 d each associated with a respective cell ofthe battery pack 2. As shown in FIG. 6, balancing circuit 6 a isconnected to CELL1, balancing circuit 6 b is connected to CELL2,balancing circuit 6 c is connected to CELL3, and balancing circuit 6 dis connected to CELL4. The positive terminal of each balancing circuitis connected to the positive terminal of the associated cell and thenegative terminal is connected to the negative terminal of theassociated cell.

In the battery monitoring and balancing system shown in FIG. 1, themonitoring system 4 has connections to the battery pack 2 that aredifferent from the connections of the balancing system 6 to the batterypack 2. In an alternative arrangement, the monitoring system andbalancing system may share connections to the battery pack 2.

Various problems can occur with such battery monitoring and balancingsystems. For example, the circuitry within monitoring system 4 thatselects which cell(s) to measure may malfunction, which may cause adifferent cell to be measured than the cell that had been selected formeasurement. For example, if CELL1 has been selected for measurement, amalfunction in the circuitry used to select which cell(s) to measure maycause another cell to be selected for measurement, such as CELL3, forexample. As a result, the monitoring system 4 may be unable to make ameasurement of CELL1, and, as a result, would not be able to detect aproblem with CELL1. The monitoring system may not be able to determinewhether CELL1 is being measured, as desired, or if another cell iserroneously being measured in its place.

Another potential problem is that the balancing system 6 may have amalfunction in the circuitry that activates balancing of a selected cellsuch that the wrong cell is activated for balancing. As a result, thecell that was selected for balancing may have a voltage that is too highor too low, and the cell that is erroneously activated for balancing maybe charged or discharged when it is not appropriate to do so,potentially imbalancing the pack, damaging these cells or reducing theirlifespan.

Another potential problem is that a balancing circuit for a cell may bestuck in the on state, continually charging or discharging the cell, orstuck in the off state, and unable to charge/discharge the cell.

It would be desirable to reduce or eliminate one or more of theseproblems to improve battery reliability.

In some embodiments, circuit elements having different characteristicsmay be individually associated with each cell of the battery pack 2 toenable discriminating cells from one another, which can allowverification that the cell being measured is the cell that has beenselected for measurement. In one embodiment, as shown in FIG. 2, eachcell is connected in series with a resistive element (e.g., a resistor).Measurements of the cell voltage may be made across the seriescombination of the cell and the resistor. In one aspect, the resistiveelements associated with the cells may have different values, whichallows identifying that the cell being measured is the cell that hadbeen selected for measurement.

FIG. 2 shows a schematic illustrating an embodiment in which resistorsare connected in series with each cell of battery pack 2. As shown inFIG. 2, a first terminal of a resistor is connected to the cell and thesecond terminal of the resistor is at a measurement node N_(i) for thecell. Both the balancing module associated with the cell and themeasurement system 4 may be connected to the second terminal of theresistive element at the measurement node N_(i). For example, as shownin FIG. 2, resistor R1A is connected in series with CELL1, resistor R1Bis connected in series with CELL2, resistor R1C is connected in serieswith CELL3, and resistor R1D is connected in series with CELL4. Both thepositive terminal of balancing module 6 a and the terminal SP1 of themeasurement system 4 are connected to measurement node N₁. The negativeterminal of the balancing module 6 a and terminal SP0 of the measurementsystem 4 are connected to the negative terminal of CELL1. A similarconfiguration is used for the other cells, as shown in FIG. 2. Themonitoring system 4 may measure the voltage for CELL 1 as the voltagebetween SP0 and SP1, the voltage for CELL 2 as the voltage between SP1and SP2, the voltage for CELL 3 as the voltage between SP2 and SP3 andthe voltage for CELL 4 as the voltage between SP3 and SP4.

In some embodiments, measurements may be made with balancing turned on,with current flowing through the series combination of the cell and theresistor, and also with balancing turned off. Such a technique enablesdetecting a malfunction in the balancing system 6.

When the balancing module is disabled, such that the current from thebalancing module is zero, the voltages measured between any given set ofsense points SP0 to SP1, SP1 to SP2, SP2 to SP3, or SP3 to SP4 is thesame voltage as measured across the cells Cell1, Cell2, Cell2 or Cell4,respectively, as long as the measurement system 4 draws low current. Inorder to measure the actual voltage on any cell, the balancing circuitmay be temporarily disabled both for that cell and the cell below it sothat there are no voltage drops on either resistor on either side of thecell to be measured. For example, if the voltage across CELL2 is beingmeasured, balancing circuits 6 a and 6 b may be disabled so there willbe negligible or zero current flowing through resistors R1A and R1B. Asa result, the voltage across resistors R1A and R1B will be zero, and thevoltage measured across terminals SP1 and SP2 will be equal to the cellvoltage.

In order to perform self-diagnosis, a measurement can be made with thebalancing module off, and a second measurement can be made with thebalancing module on. If the two measurements are equal, the differencein current flowing through the balancing circuit can be deduced to bezero, and the balancing circuit may therefore be permanently enabled orpermanently disabled.

As discussed above, the resistors R1A, R1B, R1C and R1D may havedifferent values to enable identifying the cell being measured as thecell that had been selected for measurement. Alternatively, theresistors R1A, R1B, R1C and R1D may have the same resistance values insome implementations. For example, the same resistance values may beused in an implementation where identification of individual cells isnot necessary.

To verify that all measurements are from the intended cells, the valuesof the resistors R1A, R1B, R1C, R1D may be selected to be differentenough to enable the system's software algorithm to determine whichresistor (and, therefore, which cell) is being detected. If thebalancing current to the cell is known, the voltage with balancingenabled can be compared to the voltage with balancing disabled as aratio or an expected difference. If the difference or ratio is differentthan expected, it can be presumed that the intended cell is not beingmonitored, or that the sense resistance is out of specification and afault condition can be communicated and/or other action taken.

Although resistors R1A-R1D are shown in FIG. 2 as being connected to thepositive terminals of the cells, other configurations are possible. Forexample, the resistors R1A-R1D may be connected to the negativeterminals of the cells. More than one resistor per cell may be used, insome implementations. This could require disabling balancing of the cellabove the one being measured when making a measurement.

There are several methods and configurations that can be utilized to usethe different sensed values to ensure that all cells are being measured,based (at least in part) on the level of shared circuitry involved inselecting the cell to measure, and selecting which cells to balance.

Case 1: Completely Independent Selection Circuitry for the MonitoringSystem and Balancing System.

When the monitoring system and the balancing system make separateselections of the cells, any failure in the circuitry used to selectwhich cell to measure would have no effect on any circuitry used toselect which cells to balance, and vice versa. No common select lines orother shared circuitry are present. In this case, a single value of R1could be used for all cells. When the system is selecting a given cell ito measure (where i is from 1 to N), and selecting the same cell i tobalance, the following cases of normal operation and single pointfailures are possible.

-   -   A. Cell i is still being measured but a different cell x (x≠i)        has been selected for balancing.    -   B. Cell i is stuck balancing, and cell i is being measured.    -   C. Cell i is stuck not balancing, and cell i is being measured.    -   D. Cell i balances correctly, but cell i is not being measured,        instead the monitoring system is broken and is measuring a        different cell x (x≠i)    -   E. Cell i balances correctly, and cell i is measured correctly        (no fault with the circuit)        Under all cases A through D, if a single value of R1 is being        used, the failure will be detected as follows.    -   A. For “A”, above, the same voltage is read on cell i (i.e., the        value without balancing) while a different cell other than i, x        has balancing turned on and off; the lack of voltage change for        cell i indicates a fault condition.    -   B. For “B”, above, the same voltage is read (a balancing stuck        on value) in both cases on cell i, indicating a fault condition    -   C. For “C”, above, the same voltage is read (the non-balancing        value) in both cases on cell i, indicating a fault condition.    -   D. For “D” above, a the non balancing value for cell x is read        both when cell i is balancing and when cell i is not balancing.        Reading the same value twice indicates a fault condition.    -   E. In E (no failure), a different value is read when balancing        versus not balancing showing proper operation of the circuits.

When the circuitry used to select the cell to measure is completelyindependent of the cell balancing circuitry, a single resistor value canbe used to ensure that balancing works correctly and that cellmonitoring system 4 is measuring the correct cells.

As a further tool for detection, if cell i is being measured, but celli−1 is balancing instead of cell i, this could increase the voltagereading on cell i. This condition can be detected if the voltage readingon cell i actually increases when attempting to balance i.

Case 2: Completely or Partially Dependent Circuitry Between theMonitoring and Balancing Systems.

In this case, the same circuits used to select a cell to measure arealso used to select the cell to balance. For example, if the controlsystem uses shared select binary signals, and when cell i is selectedfor measurement, the control system can also turn the balancing of cellon or off. Once the control system is done measuring cell i, it cancontinue balancing as commanded until the next time cell i is selectedfor measurement. With this architecture, a failure causing a cell otherthan cell i to be measured may cause the measured cell, rather than celli to be selected for balancing.

Going through the same possibilities as above:

-   -   A. a different cell x (x≠i) is selected for balancing, by virtue        of the shared circuitry, a different cell also x (x≠i) is also        selected for measuring,    -   B. Cell i is unable to stop balancing, and cell i is being        measured,    -   C. cell i is unable to balance, and cell i is being measured,    -   D. Cell i balances correctly, and cell i is measured correctly        (no fault with the circuit).        A single value for R1 would not be able to detect the difference        between scenario A and scenario D. Multiple values of R1 may be        needed in this case to allow differentiating scenario A from        scenario D.        If multiple values of R1 are used:    -   A) For “A” above, two voltages are read, (the non-balancing        value and the balancing value) for a cell x. If the resistor R1        on cell x is different than the resistor R1 on cell i, The        calculated value of R1 will be incorrect, and this is a        detectable error. If Cell i, and Cell x both have the same value        for R1 this would not be detected.

Note, if cell i, and cell x have the same value of R1, and the system isproperly designed, whatever failure has caused cell x to be selected andbalanced instead of cell i, will cause more cells to also be incorrectlyselected. The system will locate the failure as long as one of theinterchanged sets of cells has different values for R1. This will beexamined in further detail below.

-   -   B) For “B”, above, the same voltage is read (balancing value) in        both cases on cell i. This is a detectable error. Note: The        software may not know if this is a balancing or a non balancing        value, just that if the two readings are the same, there is an        error.    -   C) The same voltage is read (the non-balancing value) in both        cases on cell i. This is a detectable error.    -   D) A different value is read (both balancing and non-balancing        values) on cell i, and the ratio between the values or the        difference indicates that the value of R1 is correct. This        indicates that there is no error and the correct cell is being        balanced and measured.

Thus, different values for the R1 resistors may be used to ensure thatevery cell is being measured if there is shared circuitry between thesignals used to select the cell for measurement and the signals used toselect cells for balancing. The number of values needed depends on thetype of failures that are possible based on the selection architecture.

The most common types of failures to detect are when select lines arestuck high or stuck low. If an architecture has an individual selectline for every cell being measured, most common failures involve a cellthat is never selected (resulting in a reading of 0), or a cell that isalways selected (detectable by unsuccessfully trying to make a nullmeasurement).

In some architectures, multiplexers and demultiplexers are used so thatthe controller can use fewer outputs to select which cell to measure. Inthis case, a failure of a binary select signal which outputs acontinuous logic high or logic low would result in several measurementsbeing made incorrectly.

For example, consider the following table, describing a system using a16-to-1 multiplexer to select a cell for measurement with four digitalselect lines, S1, S2, S3 and S4. The value of the R1 (see FIG. 2)resistor for each cell can be selected from one of two values: Value1,Value2. It will be shown that by differentiating only a few of thepossible cells, any failure of a single line stuck high or a single linestuck low can be detected. In this particular embodiment, if theresistor chosen for R1 on cell 1 is different from the R1 chosen forcells 2, 3, 5 and 9, failures of a select line can be detected.

S4 S3 S2 S1 Selected Cell R1 Value 0 0 0 0 1 1 0 0 0 1 2 2 0 0 1 0 3 2 01 0 0 5 2 1 0 0 0 9 2S4 through S1 are select lines selecting from one of 16 cells. Resistorselected can be value 1, or value 2. There are 8 failures for the selectlines being failed in the high or low states:S4 Failed high: When trying to select cell 1, cell 9 is selected.Resistor 2 is seen instead of 1.S4 failed low. When trying to select cell 9, Cell 1 is selected.Resistor 1 is seen instead of 2.S3 failed high: When trying to select cell 1, cell 5 is selected.Resistor 2 is seen instead of 1.S3 failed low: When trying to select cell 5, cell 1 is selected.Resistor 1 is seen instead of 2.S2 failed low: When trying to select cell 1, cell 3 is selected.Resistor 2 is seen instead of 1.S2 failed low: When trying to select cell 3, cell 1 is selected.Resistor 1 is seen instead of 2.S1 failed low: When trying to select cell 1, cell 2 is selected.Resistor 2 is seen instead of 1.S1 failed low: When trying to select cell 2, cell 1 is selected.Resistor 1 is seen instead of 2.

In general, two different resistor values will be sufficient to ensurethat no select line is stuck high or stuck low. The value of R1 on cellsother than 1, 2, 3, 5 and 9 is not significant because any failure of aselect line will be caught when trying to measure cells 1, 2, 3, 5 and9. As long as all cells see a difference between the case wherebalancing is on and balancing is off, and there is a differentresistance value for CELL1 versus the values on cells 2, 3, 5 and 9, anyline stuck balancing, stuck not balancing, or a select line failure willbe caught. More resistance values can be used which can yield greaterdetail about failures as desired, or can protect from more complicatedfailures such as adjacent select lines being stuck together.

Another type of fault occurs where two select lines are stuck together.For example, for a few different faults, the table below shows thedesired cell to measure and the cell actually being measured.

Possible Cell Cell being Cell being being measured measured withmeasured with with S1 tied to S1 tied to S2 Desired Cell S4 stuck highS2 (S1 dominating) (S2 dominating) 1 9 1 1 2 10 4 1 3 11 1 4 4 12 4 4 513 5 5 6 14 8 5 7 15 5 8 8 16 8 8 9 9 9 9 10 10 12 9 11 11 9 12 12 12 1212 13 13 13 13 14 14 16 13 15 15 13 16 16 16 16 16

In order to detect failures of multiple lines stuck together, there willneed to be sufficient values of R1 distributed in a manner so that forany given detectable failure, a sequence of measurements for every cellwith everything working properly will be different than a sequence ofmeasurements with any given failure. Note, if S1 and S2 are stucktogether, in addition to S1 dominating, or S2 dominating, whichever bitis low could dominate, or whichever bit is high could dominate.

When the product goes through a design failure mode analysis, anengineer can list the errors within the selection circuitry that must bedetected, and an appropriate number of different values for R1 may bechosen to detect any failure.

If the hierarchy can be split up so that certain groups of select linescannot be shorted to other select lines, fewer values of R1 may be used.For example, if it could be guaranteed that S4 and S3 could neverinterfere with S2 and S1, four values of R1 could be used. Cells 1 to 4could be grouped together, Cells 5 to 8 could be grouped together, Cells9 to 12 could be grouped together, and cells 13 to 16 could be groupedtogether. Within each group of four cells, four different valueresistors would be used which would allow detection of any failuresbetween the S2 and S1 lines. By placing the four chosen resistor in adifferent order for each group, any failures between S4 and S3 could bedetected.

For example, in one embodiment, there are several cells grouped intosets of four cells in series called a module. There is selectioncircuitry within each module to select from one out of the four cellswithin the module to measure. There is than more selection circuitry toselect which one of seven modules is connected to a measurement device.The hierarchy is thus split into two levels, on one level one out offour cells is chosen within a module, and on the main level one of sevenmodules is selected. Furthermore, the selection lines are on differentintegrated circuits and are segregated such that failures on the selectlines for which module to select are completely independent of failuresto select a cell within a module.

Details on how to Determine which Resistance Value R1 is in the CircuitBased on Comparing the 2 Measured Values.There are two general possibilities for balancing circuits which can beused to determine the value of R1.

Case 1 Constant Current Circuits:

The first possibility is a constant current source, as illustrated inFIG. 3.

Let V_(Bal) be the voltage at the output between SP0 and SP1 with thecurrent source on at a value of I_(SRC), and let V_(Cell) be the voltagevalue with the current source at 0, or disconnected by opening switchS1.

R1 can be selected from a set of unique, discrete and predeterminedresistor values. For example, a given design may use four values, R1A,R1B, R1C, R1D.

R can be determined by: (V_bal−V_Cell)=I_(SRC)R. Furthermore, given thatthe ranges of I and R, as well as possible uncertainties in themeasuring circuit, are known ahead of time, a range between two voltagesV_(Min) _(R1D) and V_(Max) _(R1D) can be determined such that:

If V_(Min) _(R1D)<=V_(bal)−V_(Cell)<=V_(Max) _(R1D), the resistor valueis R1D. These ranges can be precalculated for the different selectedvalues of R1, the tolerances on R1 and the ranges for the currentsource. Depending on the range/tolerance of the values of R1, thesensitivity of the voltage measurement and the range/tolerance of thecurrent source, the different values of R1 will have to have values thatare sufficiently different so that the ranges do not overlap.

This is equally valid in the case where the switch, S1, is a solid-stateor semiconductor device which has a non-zero voltage (sometimes referredto as the saturation voltage) across it while it is closed, and/or anon-zero current through it while it is open (sometimes referred to asleakage current). The technique of worst-case circuit analysis, known tothose skilled in the art, can be used to design the circuit and selectcomponent values to allow the resistor choice to be determinedunambiguously.

Case 2 Constant Voltage Circuits:

The second possibility, as illustrated in FIG. 4, is two resistors beingconnected to a constant voltage source V_Device. These values are R1 andR2. The voltage source (V_Device) can represent any component that couldbe analyzed as a constant voltage source. For example, V_Device can beclose to 0V if a switch is used to connect the resistors to the negativeof the cell, or V_Device can be a transistor voltage drop if using atransistor to connect R2 to the negative point of the cell, or V_Devicecould be a charging voltage source which is higher than the cell voltagewhich would charge up the cell through R2 and R1, finally V_Device couldbe a discharging constant voltage lower than the cell voltage such asseries diodes designed to discharge the cell towards a dischargevoltage.

In this case, R1 and R2 can be chosen to add up to a constant sum sothat even with different values of R1, the total balancing resistanceremains the same from cell to cell. The equations governing thissituation are:

Let V_Cell be the voltage measured between SP0 and SP1 with S1 openwhich is also the voltage across Cell1Let V_Bal be the voltage measured between SP0 and SP1 with S1 closed.

V_bal−V_Cell=I*R1.

I=(V_Cell−V_Device)/(R1+R2)

If I, V_Cell, V_device, and the tolerance for a given R1 and R2 varylittle enough, and the chosen values of the different R1 resistors arefar enough apart, ranges can be precalculated as with the first case. Ifthe values vary too much (especially if V_Cell as it ranges from fullydischarged to fully charged), the ranges can be calculated during systemoperation based on the measured V_Cell and known ranges for V_Out, R1and R2.

Variations of Resistor-Based Circuits:

The added resistors from FIG. 2, R1A to R1D could be placed on eitherthe positive or negative connection to the cells in question, andvoltage or current sources could then be placed on the other side of theresistors. In order to check the function of the balancing, two readingsare needed. For simplification, above an embodiment is described inwhich the current is nominally zero for one of the two readings (anon-balancing reading was compared with a balancing reading). However itis possible to determine if the correct value of the R1 resistor isconnected without one of the two readings being with zero current. Forexample, if discharging the cells through a controllable dischargecurrent source, one reading could be under a small discharge current,and the 2^(nd) reading could be under a heavier discharge current.Another example would be charging through a controllable voltage sourcewith one reading set to a low charge voltage and the 2^(nd) reading setto a higher charge voltage. By comparing the difference between tworeadings under two different operating conditions, the value of R1 canbe determined. In most cases one of the two readings would be under 0current, but it is possible to make two readings both with differentcurrents flowing through R1 and based on the difference in the currentsto be able to determine R1.

Note, FIGS. 3 and 4 both show switches, but if a current source can becommanded to zero, or to multiple different values, or if the voltagesource can be commanded to different values, a switch would not beneeded to get the two different required readings to determine that R1is within the required range.

Further Variations

Above has been described an embodiment in which resistive elementsassociated with each cell have different resistance values to enableidentifying a cell that has been selected for measurement. In someembodiments, the cells may be discriminated from one another using adifferent technique. For example, each cell may be connected to aresonant circuit that is configured to resonate at a differentfrequency. When making a measurement of the cell, an AC signal may beapplied to the cell, and an AC measurement may be made to identifywhether the resonant circuit associated with the cell resonates at theexpected frequency. Alternatively, each cell may be connected to abandpass or bandstop filter that is configured to pass, or block,respectively, a different frequency range. In such a case, an ACmeasurement may be made to check the filter associated with a cell hasexpected frequency characteristics. In another example, a digitalcircuit may be associated with each cell having differentcharacteristics, such as different stored digital values. Thus, varioustechniques may be used to discriminate cells from one another based onelectrical characteristics. In such techniques, circuit elements can beindividually associated with respective electrochemical cells of thepack and have electrical characteristics that are different enough suchthat individual electrochemical cells can be distinguished from oneanother. However, the techniques described herein are not limited toidentifying cells electrically, as any other suitable technique can beused to identify cells.

Example

The embodiment shown in FIG. 5 and FIG. 6 and is built on top of anarchitecture as follows: there is monitoring and balancing circuitry fortwenty-eight cells. There are seven groups of four cells in seriestotaling the twenty-eight cells. The monitoring and balancing circuitryfor four cells in series is called a module. Thus, there are sevenmodules each monitoring and balancing four cells.

As shown in FIG. 5, each module has multiplexers 51 to select betweenfour cells, and serial digital data may be sent to each module tocontrol the balance data and select lines for that module. The resistorsR1 are connected to the negative terminals of the cells, there are fourdifferent values of R1. Each module may include a logic and isolationmodule 53 that may include logic to control the multiplexers to providemeasurements to the measurement system and to control the balancingcircuits, as well as circuitry to isolate the module. In this example,the balancing circuit for each cell includes a PNP transistor.

FIG. 5 shows one module for four cells, and FIG. 6 shows the connectionof all seven modules together. FIG. 6 shows the seven modules connectedtogether to make a device capable of measuring twenty-eight cells. Thereis a decoder/demultiplexer 61 which uses three select lines to selectone of the 7 modules in response to commands from the controller 62(which may have a CPU or other processor). The modules are selected whenswitches connect an ADC (analog-to-digital converter) and buffer circuit63 of the measurement system to the module in question while all othermodules are disconnected from the ADC.

Relating FIGS. 5 and 6 to FIG. 1, the multiplexers the switches, thefilters and the ADC are all considered part of the monitoring system 4.The balancing PNP transistors and R2 resistors are part of the balancingsystem 6. The R1 resistors are the R1s listed in FIG. 1 for theself-check. The logic and isolation block 53 within each module, thecontroller 62 and the decoder 61 are all part of both the monitoring andbalancing system.

The following failures may be detected:

-   -   1) Any failure of the select lines within a module (including        stuck high, or stuck low or stuck together)    -   2) Any failure of a balancing transistor stuck on or stuck off.    -   3) Any failure of the select lines to the decoder. (including        stuck high, or stuck low or stuck together)

Note: Decoder outputs stuck high or low, or switches stuck on or off aredetected by using methods known to the art including zeroing themeasurement circuitry between measurements and also by attempting toNull measurements with no modules connected.

Decoder outputs stuck high or low, or switches stuck on or off aredetected by doing the following:

Measurements may be made when the decoder is selecting none of themodules. If a valid voltage is read, one of the modules is actually oneither through a decoder pin stuck high fault, or through a switch stuckon fault, and a DTC will be set.

Also, measurements may be set to zero before connecting a module. If avoltage should make a valid reading but reports zero instead, a DTC isset which may indicate a decoder line stuck low, or a switch stuck off.Items 1 and 2 from above may be detected as follows:

The resistor values R1 are set up so that each cell within a givenmodule has a unique value of R1, selected from four values, R1_(—)1,R1_(—)2, R1_(—)3, R1_(—)4. This ensures that if select signals S1 and/orS2 within a module experience fault conditions, the fault within thatmodule will be detected.

The values of R1 and R2 associated with each cell add up to a constantvalue. Relating FIG. 5 to FIG. 4, the voltage source V_(device) in FIG.4 is the voltage drop of the PNP transistor in FIG. 5. This uses theconstant voltage method of identifying the identification resistor.

The voltage output of the module is connected to a measurement devicedepending on the value of the SW_CTRL signal. Faults in the SW_CTRLsignal must also be detected. This is done by ensuring that the orderingof the 4 different values of R1 is different within each module.

If the values of R1 are R1_(—)1, R1_(—)2, R1_(—)3, R1_(—)4, thefollowing table shows one example of the sequencing to identify modules

Resis- Module Module Module Module Module Module Module tors 1 2 3 4 5 67 R1A R1_1 R1_2 R1_3 R1_4 R1_1 R1_3 R1_2 R1B R1_2 R1_3 R1_4 R1_1 R1_3R1_2 R1_4 R1C R1_3 R1_4 R1_1 R1_2 R1_4 R1_1 R1_3 R1D R1_4 R1_1 R1_2 R1_3R1_2 R1_4 R1_1

Accordingly, if an unexpected value is found for the presumed resistorR1 during the sequencing among the cells within a given module, there isa fault within the module. Moreover, if four unique values are foundwithin a module, but the values not in the expected order, an incorrectmodule has been selected. Using more than the minimum of two uniqueresistor values in this case allows for increased diagnostic ability indetermining what the failure may be. The embodiment described andillustrated above can check if the select lines from the CPU are stuckhigh or low by ensuring that the correct balancing ratios are detectedin the correct order in each module. By taking a reading with no bankselected, it can be ensured that none of the enable lines to any moduleare stuck high. If any enable line to any of the modules is faulted low,0V readings will be made when that module is selected, and thiscondition will accordingly also be detected. Within each module,failures of the multiplexers, or the S1/S2 select signals are detectedusing the balancing ratios. Finally, any channel with balancing stuck onor off is detected. By using the disclosed methods, control signalswhich are faulted high or low or stuck together, module enable linesfaulted high or low, multiplexer select lines faulted high or low orstuck together, or balancing faulted on or off is detected for thiscircuit.

Adjacent Cells

In some cases, a current or voltage applied to one cell may affect themeasurement for an adjacent cell. For example, if a cell is stuckbalancing, such that current is continually applied to the cell, thevoltage measurement for the adjacent cell may be affected. For example,a current provided through the series combination of the resistor andcell would create a voltage drop across the resistor, which changes thevoltage measured at the adjacent cell, as the measurement is made acrossthe shared resistor. If a cell is stuck balancing, the voltage that ismeasured for the adjacent cell may be lower than the true voltage of thecell, due to the voltage drop across the resistor.

The above can be used when making measurements to determine informationregarding the cells. As discussed above, measurements may be made for acell with balancing turned on and with balancing turned off. The voltagemeasured with balancing turned on should be different than themeasurement made with balancing turned on, due to the voltage dropacross the resistor. If they are the same, then balancing for the cellis either stuck on or stuck off. If the voltage difference between themeasurement with balancing on and balancing off has the wrong sign(i.e., the ratio between the two is >1), this may be due to an adjacentcell that is stuck balancing. If the ratio is less than one, but theratio is too high, two cells may be inadvertently measured at the sametime or the wrong channel may be selected. If the ratio is too low, thewrong channel may have been selected or there may be high senseresistance Rs in the connection to the measurement circuit (shown inFIG. 5).

Measurement of and Compensation for Sense Resistance

In some embodiments, the measurement system 4 may be configured tomeasure the sense resistance(s) Rs for a connection. Measuring the senseresistance Rs may assist in identifying a sense resistance that is toohigh, for example, or may enable taking into account the senseresistance when checking measurements for the cell.

Refer to FIG. 5, Cell 4. If the cell has no source resistance, themeasured voltage with balancing=Voltage without balancing−R1D*(Voltagewithout balancing−Transistor Drop)/(R1D+R2D)

Now, if there is resistance in the cell, or the cell wiring, it can beshown that the equation changes to:

the measured voltage with balancing=Voltage without balancing−(R1D+Senseresistance)*(Voltage without balancing−Transistor Drop)/(R1D+R2D+SenseResistance)

If other quantities are well known, the sense resistance can becalculated.

In some embodiments, a wetting current may be applied to the connectionsbetween the measuring system 4 and the measurement points to reduce thesense resistance. For example, such a wetting current may be applied bythe balancing system 6. Such a current may be applied to reduce thesense resistance when it is determined that the sense resistance is toohigh.

Time Rate of Change Measurements

In some embodiments, measurements may be made of the voltage of a cellover time as the cell is charging or discharging. Making measurements ofthe voltage of a cell as the voltage changes can provide informationregarding the state of charge (SOC) for the cell.

FIG. 7 illustrates an exemplary plot of cell voltage vs. state ofcharge, which is the percentage of charge stored in the cell relative toits charge storage capacity. As shown in FIG. 7, the change in voltageis steeper at the upper and lower extremes of SOC, and less steep atmoderate SOC values. In other words, when a cell is discharging orcharging, the voltage of the cell will change more rapidly (for a givencurrent) when the SOC is either a low SOC value or a high SOC value. Bymaking measurements over time as the cell is charging or discharging, itcan be determined whether the cell has a moderate SOC, or an SOC closerto the extremes. This information can be helpful in variouscircumstances. For example, if the measurement system has both anaccurate measurement unit and a less accurate back-up, time rate ofchange measurements may be useful if the accurate measurement unitfails. If only a coarse measurement is available, time rate of changemeasurements can be useful to determine whether the cell has a moderateSOC or a more extreme SOC.

The ability to use rate of change with a coarse measurement to determineSOC can be used to distinguish a channel with balancing stuck on from achannel with balancing stuck off. If balancing is stuck on, themeasurement will be consistently low. If balancing is stuck off, themeasurement will be accurate. If it is not known whether balancing isstuck on or off, the measurement that results could be inaccurate. Usingtime rates of change can allow the system to determine whether the SOCis high or low which can then allow the system to determine if thevoltage measured is accurate (and therefore balancing is stuck off), orif it is too low (and therefore balancing is stuck on).

Further Discussion

By putting resistors into a battery measuring and balancing circuit, andby measuring the voltage drop across these resistors with and withoutbalancing enabled, with a known balancing current, the resistor valuecan be calculated during system operation. By ensuring unique resistorvalues and/or value sequences, the balancing circuit can be used toensure that the monitoring and balancing circuits are both functioningproperly on every cell. Readings with and without balancing enabled arecompared, and this can be used to ensure that the correct cells arebeing measured and that balancing is functioning as commanded. If thebalancing selection circuitry is completely independent from themonitoring selection circuitry, a single resistor value suffices tocheck that everything is working. Further values of R1 could helpprecisely locate errors for diagnostics but are not necessary. If thereare shared aspects of the balancing selection and monitoring selectioncircuitry, multiple values of R1 are needed to ensure that no faultsexist in the system. Two values of R1 suffice to detect failures ofsingle select lines faulted stuck at logic one, or faulted stuck atlogic zero. If the failures are more complex (for example select linesstuck together), more values of R1 can be used and distributed such thatfor any given failure, the self check results would be different forthat failure versus a fully functioning system.

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific inventive embodiments described herein. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, inventive embodiments may be practicedotherwise than as specifically described and claimed. Inventiveembodiments of the present disclosure are directed to each individualfeature, system, article, material, kit, and/or method described herein.In addition, any combination of two or more such features, systems,articles, materials, kits, and/or methods, if such features, systems,articles, materials, kits, and/or methods are not mutually inconsistent,is included within the inventive scope of the present disclosure.

For example, embodiments of controllers, such as controller 62, may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable hardware processor or collection of hardware processors,whether provided in a single computer or distributed among multiplecomputers. It should be appreciated that any component or collection ofcomponents that perform the functions described above can be genericallyconsidered as one or more controllers that control the above-discussedfunctions. The one or more controllers can be implemented in numerousways, such as with dedicated hardware, or with general purpose hardware(e.g., one or more processors) that is programmed to perform thefunctions recited above.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

The various methods or processes outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, various inventive concepts may be embodied as acomputer readable storage medium (or multiple computer readable storagemedia) (e.g., a computer memory, one or more floppy discs, compactdiscs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other non-transitory medium or tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of embodiments as discussedabove. Additionally, it should be appreciated that according to oneaspect, one or more computer programs that when executed perform methodsof the present invention need not reside on a single computer orprocessor, but may be distributed in a modular fashion amongst a numberof different computers or processors to implement various aspects of thepresent invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Note that the actual embodiment may be realized using discreteelectronics, integrated circuits or the construction of the most or allof the entire system on a single application-specific integrated circuit(ASIC) specifically for this application.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

What is claimed is:
 1. A method of monitoring an electrochemical cell,the method comprising: measuring a change in voltage over a time periodas the electrochemical cell charges or discharges; and determining,based at least partially on the change in the voltage over the timeperiod, a range of the state of charge of the electrochemical cell. 2.The method of claim 1, wherein, when the change in the voltage over thetime period is higher than a threshold, the range is determined tocorrespond to a low state of charge or a high state of charge.
 3. Themethod of claim 2, wherein, when the change in the voltage over the timeperiod is lower than a threshold, the range is determined to correspondto a moderate state of charge between the low state of charge and thehigh state of charge.
 4. The method of claim 2 further comprisingidentifying which of two or more possible voltage readings for theelectrochemical cell is correct based upon the determined range of thestate of charge for the electrochemical cell.
 5. The method of claim 4,wherein the two or more possible voltage readings are produced as aresult of a fault within measurement, balancing and/or selectioncircuitry.
 6. A system for monitoring an electrochemical cell, thesystem comprising: a measurement circuit configured to measure a changein voltage over a time period as the electrochemical cell charges ordischarges; and a controller configured to determine, based at leastpartially on the change in the voltage over the time period, a range ofthe state of charge of the electrochemical cell.